insert on duplicate update?

hi,
is there in postgres anything similar to:
insert .... on duplicate update?
here is the problem i need a solution for:
I need to insert in pg_type a row which was previously selected from there
and
with slight modifications, the problem is that when i try to insert the row
there might be a row with the same 'typname' which is sort of primary key.
The second problem is that when doing those operations i don't want to
depend
on the specific structure of the pg_type catalog. I want my procedure
to work even if in some future release additional columns to pg_type are
added.
So what i want is to insert a row in a table that was previously selected
 from the same
table but if there is primary key collision i want to update the row that
collided. And
all these without being dependant on the table structure.

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